1. Field of the Invention
The present invention relates to the control of ventilation air in heating, ventilating and air-conditioning systems and more particularly to a system whereby the volume of outside air heated by the system is reduced as the outside temperature drops.
2. Description of the Prior Art
In heating, ventilating and air-conditioning systems, it has become a normal practice to introduce at least a minimum volume of fresh air into an occupied room for ventilation purposes. This practice assures that in recirculation type systems the air being discharged into a room contains a minimum percentage of fresh air with the remaining air being recirculated air from the room. This practice is so desirable that many governmental agencies have established ventilating codes requiring that a certain percentage of the air being discharged into an occupied room be fresh air. In order to conform to these ventilation codes, prior art heating, ventilating and air-conditioning systems set the outside and recirculation air dampers so that the dampers could not be moved to a position that would allow less than the minimum code requirement of fresh air when the system was in operation.
While providing a minimum volume of fresh air is desirable, the cost of operating such a system is rapidly becoming prohibitive during cold winter weather. The outside air must be heated to the discharge air temperature and on cold days, the energy required to heat the outside air to discharge air temperature is a large percentage of the total energy required for heating a room. Thus, any excess outside air that is heated and discharged into the room becomes a major source of wasted energy which, in turn, results in higher operating costs. Thus, the prior art devices have allowed the outside air dampers to close to the minimum position to meet code requirements without any further control over the damper operation.
The prior art devices have ignored the important Charles Law relationship between air volume and temperature. By ignoring this relationship a significant waste of energy is experienced since an excess volume of outside air is heated to discharge air temperature. As the outside air is heated from low outdoor air temperatures to the discharge air temperature, the volume is significantly increased and the volume increase gets larger as the outdoor temperature drops. Thus, less outside air is required at low temperatures than at higher temperatures in order to achieve a constant volume or percentage of fresh air being discharged into the room.
As an example, consider a heating and ventilating system that provides 1000 cubic feet per minute of discharge air at a temperature of 70.degree. F. and a minimum of 25% fresh air. Each minute 250 cubic feet of outside air is drawn into the system and heated to 70.degree. F. If the outside air temperature is 0.degree. F., the 250 cubic feet of outside air will increase to 300 cubic feet during the temperature rise to 70.degree. F. Thus, 50 extra cubic feet of air is heated from 0.degree. to 70.degree. F. during each minute of operation or an extra 72,000 cubic feet of air is heated per day. Thus, it is clear that a substantial waste of energy is experienced by the prior art devices.